The present invention relates to a headbox or breastbox for a papermaking machine, and particularly to means for adjusting the pulp density or concentration of the pulp suspension over the working width of the headbox or the machine width. One such headbox is known from Federal Republic of Germany Patent 35 14 554 equivalent to U.S. Pat. No 4,88,094. Such a headbox is intended to make the pulp suspension uniform over the entire cross machine width of the pulp outlet from the headbox. At the downstream end of the flow path of the suspension, it should be made uniform in front of the discharge or outlet slot from the headbox. The uniformity sought is such that both the density of the pulp, that is, the weight of fiber content per unit volume, and the orientation of the fibers in the pulp, are constant over the width of the pulp outlet from the headbox. Both of these qualities are important prerequisites for the finished paper being produced by the papermaking machine, in order to have a proper weight per unit area profile over the entire cross machine width so called basis weight cross profile of the web and so that the paper lies flat and does not tend to curl.
During operation of the papermaking machine, numerous disturbing factors interfere with the satisfaction of the two uniformity requirements. These factors include temperature variations, pressure variations and manufacturing tolerances in the headbox and in the pulp suspension, for example.
The above noted German patent is concerned with solving the same problems as are noted above, which are also the problems to be solved by the present invention. That patent recognizes that it is important both to maintain the density of the fibers in the pulp suspension over the width of the pulp outlet and also to control the fiber orientation so that, if it is possible, no transverse flow will occur in the outlet channel. The German patent proposes that the density of the pulp suspension be changed locally, that is that the density of the pulp suspension be changed at given places across the machine width, as required. However, the patent does not provide what is believed by the present inventors to be the best solution to this problem.
It is also known to vary the width of the discharge slot, that is, the height of the outlet opening at the discharge slot. One way to do that is by the use of threaded spindles for swinging or bending one lip, and particularly, the upper lip that defines the discharge slot. For instance, see Federal Republic of Germany Patent 29 42 966, corresponding to U.S. Pat. No. 4,326,916, or Federal Republic of Germany Published Application OS 35 35 849. This adjustment of the width of the discharge slot enables local variation of the throughput of the suspension. At the same time, however, the direction of suspension flow is also locally affected, which affects the orientation of the fibers in the suspension. The local narrowing of the outlet slot causes a different flow direction in the fibers at the narrowed places of the slot than along the remainder of the discharge slot. Although the density of the pulp can be made uniform over the width of the pulp outlet by the so-called displacement control, the fiber orientation, which may have been good, is undesirably again disturbed. Although the inventors have recognized that the last two above noted German patent applications proceed fundamentally in the correct direction, nonetheless, they do not appear to be able to control independently the two parameters of the density of the pulp and the fiber orientation.